Parallax-reducing, luminance-preserving diffuser

ABSTRACT

A diffuser incorporates diffuser elements ( 10 ) that each include a waveguide ( 12 ) coupled between a concentrator ( 14 ) and an inverse concentrator ( 16 ). The diffuser ( 44 ) may be disposed between a backlight ( 102 ) and a modulator ( 114 ) in a display ( 100 ). Luminance ratios of light rays emitted by a low resolution image-forming backlight ( 102 ) toward a high resolution light modulator ( 114 ) may be preserved such that, for viewing directions within the display&#39;s preferred angular viewing range, an observer perceives no significant change in the luminance of displayed images, irrespective of changes in the observer&#39;s viewing direction.

CROSS REFERENCE TO RELATED APPLICATION

This application claims to benefit U.S. application No. 60/591,088 filed27 Jul. 2004 and entitled “PARALLAX-REDUCING, LUMINANCE-PRESERVINGDIFFUSER”.

TECHNICAL FIELD

This invention pertains to optics and more particularly to the optics ofdisplays, such as computer displays, television displays and the like.The displays may be LCD displays. The invention can be applied to highdynamic range displays as well as displays of other types.

BACKGROUND

In a typical high dynamic range display, a backlight produces acomparatively low resolution image on an inward side of a lightmodulator. The low resolution image is modulated by the light modulatorto provide a comparatively high resolution image, which appears on theoutward side of the light modulator for perception by an observer. Themodulator may comprise an LCD. Electronic signals for controlling thebacklight and the light modulator may be generated by suitable controlcircuits of known types. For example, an LCD light modulator may becontrolled using control circuits of the same type used to control theLCD modulator in a conventional LCD computer monitor. Example highdynamic range displays are disclosed in international patent publicationWO 02/069030 published 6 Sep. 2002 and in international patentpublication WO 03/077013 published 18 Sep. 2003, both of which arehereby incorporated by reference herein.

Multiple light sources within the backlight may illuminate each pixel ofthe light modulator. Maintenance of a relatively small separationdistance between the backlight and the modulator allows adjacent pixelsof the low resolution image to merge smoothly into one another. Wellknown image compensation techniques may be applied to remove anyundesirable image-blurring artifacts.

One difficulty with such image compensation techniques is that the lightintensity (luminance) distributed from a pixel of the low resolutionimage to a corresponding pixel of the high resolution image is ideallyinvariant. Otherwise, the intensity of the high resolution image'spixels may vary as a function of the direction from which the image isviewed by an observer, which is undesirable. One common method ofachieving such luminance invariance is to incorporate in the display adiffuser having a Lambertian output distribution (i.e. the angulardistribution of light rays emanating from the outward side of thediffuser is symmetrical about the diffuser's normal direction and isindependent of the direction of the corresponding incident light rays).Such diffusers eliminate parallax (apparent changes in the direction ofan object, due to changes in the observer's position which correspond todifferent lines of sight to the object). Accordingly, the observer doesnot see unwanted images of things behind the display layer.

Lambertian diffusers undesirably reduce the display's overall luminanceby a factor of about ten. This is partly due to backscatter of lightrays within the diffuser, and partly due to the diffuser's inherentfunction of spreading luminous flux over a relatively large solid angle(effectively π steradians, compared to a preferred solid angle of nomore than about 0.5 steradians).

The inventors have recognized a need for display apparatus thatpreserves image luminance in display backlights by confining incidentlight rays within a preferred angular viewing range, in a manner whichreduces viewing parallax by restricting the dependence of the angulardistribution of light rays transmitted through the display on thedirection from which an observer views the display.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended non-limiting drawings:

FIG. 1 is a cross-sectional side elevation view, on a greatly enlargedscale, of a diffuser element.

FIG. 2 is a cross-sectional top plan view, on a greatly enlarged scale,of a fragmented portion of a diffuser sheet incorporating a plurality ofthe FIG. 1 diffuser elements.

FIG. 3 is a front elevation view, on a greatly enlarged scale (andlarger than that of FIG. 2), of a fragmented portion of the FIG. 2diffuser sheet.

FIG. 4 is a schematic cross-section through a portion of a displayincorporating a diffuser sheet like that of FIG. 2.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

FIG. 1 shows a single diffuser element 10. A diffuser suitable, forexample, for use in diffusing light incident from a backlight on amodulator in a display can include a large number of diffuser elementsdistributed over the diffuser. The display may comprise a high dynamicrange display or another type of display such as a conventional LCDdisplay, for example.

Diffuser element 10 includes a cylindrical waveguide 12 opticallycoupled between a compound parabolic concentrator 14 and an inversecompound parabolic concentrator 16. Waveguide 12 may be formed of anysuitable material having an index of refraction sufficiently dissimilarto that of the surrounding material (e.g. air) that light rays aretotally internally reflected within waveguide 12. For example, waveguide12 may be formed of a suitable transparent or dielectric material suchas a suitable optically transparent glass or plastic material.

Compound parabolic concentrator 14 concentrates incident light rays 18,20, 22 and directs light rays from compound parabolic concentrator 14into waveguide 12. Before incident light rays 18, 20, 22 enter compoundparabolic concentrator 14, a collimator (not shown in FIG. 1) collimatesrays 18, 20, 22 such that, for an air medium, rays 18, 20, 22 lie withina preferred angular viewing range, namely within about 25° of thedisplay's normal direction indicated by arrow 24. Collimation maximizesthe number of light rays which enter compound parabolic concentrator 14,thus maximizing the number of light rays which are coupled intowaveguide 12.

Compound parabolic concentrator 14 and inverse compound parabolicconcentrator 16 may be formed of any suitable material or materialshaving an index of refraction sufficiently dissimilar to that of thesurrounding material (e.g. air) that light rays are totally internallyreflected at their boundaries. Compound parabolic concentrator 14 andinverse compound parabolic concentrator 16 may each be made of the samematerial as waveguide 12. For example, compound parabolic concentrator14 and inverse compound parabolic concentrator 16 may be made ofsuitable transparent or dielectric materials such as suitable opticallytransparent glass or plastic materials. In some embodiments, compoundparabolic concentrator 14, inverse compound parabolic concentrator 16and waveguide 12 are all made of the same material. This avoidsreflections at interfaces between compound parabolic concentrator 14,inverse compound parabolic concentrator 16 and waveguide 12.

Waveguide 12 totally internally reflects and spatially homogenizesconcentrated light rays 26, 28, 30 before they pass into inversecompound parabolic concentrator 16. In this context, “spatialhomogenization” means that information characterizing the direction oflight rays entering waveguide 12 is substantially removed. Spatiallyhomogenized light rays 32, 34, 36 are directed out of diffuser element10 by inverse compound parabolic concentrator 16 (which may be acompound parabolic collimator). Light rays 32, 34, 36 emerge fromdiffuser element 10 as corresponding rays 38, 40, 42. Inverse compoundparabolic concentrator 16 may restore the light emitted from diffuserelement 10 to a level of collimation similar to or the same as that ofthe light incident on diffuser element 10.

Compound parabolic concentrators are well known, well-defined opticaldevices. For example, see Structure for Efficiently Coupling Large LightSources in Prism Light Guides, P. Kon et al., Journal of theIlluminating Engineering Society, p. 78-82, 2000. The diameter ofwaveguide 12 is selected to match the diameters of concentrators 14, 16where they respectively intersect waveguide 12. The length of waveguide12 is not critical. Waveguide 12 is preferably sufficiently long thatlight rays are totally internally reflected a significant number oftimes (e.g. 2 or more and preferably 3 or more times or even 10 or moretimes) as they pass through waveguide 12 in order to attain theaforementioned spatial homogenization. This generally means thatwaveguide 12 has a length that is at least twice its diameter (ifwaveguide 12 is round in cross section) or its largest transversedimension (if waveguide 12 is not round in cross section).

FIGS. 2 and 3 depict a portion of a diffuser sheet 44 incorporating alarge plurality of diffuser elements 10. Sheet 44 preserves imageluminance by maximizing the likelihood that a light ray emanating from apixel of the low resolution image will reach and contribute to theluminance of a corresponding pixel of the high resolution image. Byspatially homogenizing light rays which pass through each diffuserelement 10, sheet 44 also substantially removes those rays' incidentangular distribution characteristic, within the desired 25° collimationangle. Consequently, the luminance ratios of light rays passing from thelow resolution image to and forming the high resolution image, areperceived by persons observing the display to be substantiallyindependent of the observer's viewing direction, within the desiredviewing range. This facilitates accurate application of any suitableimage compensation techniques to remove undesirable artifacts such asblurring artifacts.

FIG. 4 shows a display 100 that incorporates a diffuser sheet 44.Display 100 includes a backlight 102. In the illustrated embodiment,backlight 102 comprises an array of light emitting diodes (LEDs) 104.Light emitted by LEDs 104 passes through a collimator 108. Collimator108 is preferably of a type that “recycles” light that is not travelingwithin the angular range that is passed by the collimator. Collimatedlight passes through waveguides 12 (not shown in FIG. 4) of sheet 44 andimpinges on pixels 112 of a light modulator 114. Light modulator 114 maybe an LCD panel or other transmission-type light modulator, for example.

In the illustrated embodiment a controller 120 comprising a graphicsdata processor 122 and suitable interface electronics 124A forcontrolling backlight 102 and 124B for controlling light modulator 114receives image data 126 specifying images to be displayed on display 100and drives the light emitters (e.g. LEDs 104) of backlight 102 and thepixels of light modulator 114 to produce the desired image for viewingby a person or persons. Controller 120 may comprise a suitablyprogrammed computer having appropriate software/hardware interfaces forcontrolling backlight 102 and light modulator 114 to display an imagespecified by image data 126.

Typically diffuser elements 10 are much smaller in diameter than thepixels of a high resolution light modulator that they illuminate. Eachpixel of the high resolution modulator may be illuminated by lightpassing through a plurality of diffuser elements 10. For example,diffuser elements 10 may have diameters of 10's of microns (i.e. in therange of about 10 microns to about 100 or 200 microns) while each pixelof a high resolution modulator may have dimensions on the order of 100'sof microns (e.g. typically in excess of about 100 microns). A singlepixel of a high resolution modulator may be illuminated by light thathas passed through 10 or more diffuser elements 10. In some embodiments,each diffuser element 10 is sized to correspond to the size of one pixelof the display's high resolution image.

Where a component (e.g. a member, part, assembly, device, processor,controller, collimator, circuit, etc.) is referred to above, unlessotherwise indicated, reference to that component (including a referenceto a “means”) should be interpreted as including as equivalents of thatcomponent any component which performs the function of the describedcomponent (i.e., that is functionally equivalent), including componentswhich are not structurally equivalent to the disclosed structure whichperforms the function in the illustrated exemplary embodiments of theinvention.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the scope thereof.For example:

-   -   Although described above in relation to high dynamic range        displays, the invention is of general application and can be        used in conjunction with any direct backlight display        sub-system, such as a liquid crystal display (LCD). Such an LCD        may be backlit, for example, by an array of individual light        sources such as an array of red-green-blue Luxeon DCC™ light        emitting diode light sources available from Lumileds Lighting,        U.S. LLC of San Jose, Calif.    -   Waveguide 12 and concentrators 14, 16 need not be circular in        cross-section, but may have other cross-sectional shapes. For        example: waveguide 12 may be rectangular or hexagonal in        cross-section; concentrators 14, 16 may be conically shaped.        Such alternative shapes are less optically efficient than the        shapes described above but may be acceptable in some        applications where it is desirable to make a diffuser that is        easier and less expensive to fabricate.    -   Diffuser elements 10 need not be laid out in square grid fashion        as shown in FIG. 3, but may have any other desired layout (e.g.        random distribution, hexagonal, rectangular grid, etc.)        compatible with the layout of the pixels or pixel groups of the        display sheet.    -   It is not mandatory that parabolic concentrator 14 and inverse        compound parabolic concentrator 16 are the same size as one        another although both will typically by of the same, or a        similar, size to match waveguide 12.    -   Instead of providing structures in which light is totally        internally reflected, parabolic concentrator 14, inverse        compound parabolic concentrator 16 and/or waveguide 12 may be        made by providing reflective coatings or by providing a        reflective material such as a reflective metal surrounding        parabolic concentrator 14, inverse compound parabolic        concentrator 16 and/or waveguide 12. In such embodiments the        parabolic concentrator 14, inverse compound parabolic        concentrator 16 and/or waveguide 12 may be provided in the form        of a hollow structure with reflective walls.

While a number of example aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truescope.

1. A diffuser sheet having parallel opposing spaced apart light-incidentand light-emitting sides, the diffuser sheet comprising a plurality ofdiffuser elements, each diffuser element comprising: a concentratorpositioned in the diffuser sheet toward the light-incident side toreceive light incident on the diffuser sheet; an inverse concentratorpositioned in the diffuser sheet toward the light-emitting side whereinthe concentrator and inverse concentrator are spaced apart from oneanother; and an internally-reflecting waveguide optically coupledbetween the concentrator and the inverse concentrator the waveguideconfigured to carry light from the concentrator to the inverseconcentrator.
 2. A diffuser sheet according to claim 1, wherein theconcentrator is a compound parabolic concentrator.
 3. A diffuser sheetaccording to claim 1, wherein the inverse concentrator comprises aninverse compound parabolic concentrator.
 4. A diffuser sheet accordingto claim 2, wherein the compound parabolic concentrator concentratesincident light rays to maximize the number of light rays passing fromthe compound parabolic concentrator into the waveguide.
 5. A diffusersheet according to claim 4, comprising a collimator for collimating theincident light rays, before the incident light rays enter the compoundparabolic concentrator.
 6. A diffuser sheet according to claim 5,wherein the collimator causes the incident light rays to be directedwithin about 25° of a direction normal to the diffuser sheet.
 7. Adiffuser sheet according to claim 5, wherein the waveguide decollimatesthe concentrated light rays and optically couples the decollimated lightrays into the inverse compound parabolic concentrator.
 8. A diffusersheet according to claim 7, wherein the inverse compound parabolicconcentrator collimates the spatially homogenized light rays tosubstantially the same extent that the collimator collimated theincident light rays.
 9. A diffuser sheet according to claim 1 whereinthe waveguide comprises an optically-transparent dielectric materialsurrounded by air.
 10. A display comprising a diffuser sheet accordingto claim 1 located between a backlight and a light modulator.
 11. Adisplay according to claim 10 comprising a collimator between thebacklight and the diffuser sheet.
 12. A display according to claim 11,wherein the collimator causes the majority of light incident on thediffuser sheet to be directed within a viewing angle of 25° to a lineperpendicular to the diffuser sheet.
 13. A display according to claim 10wherein the backlight comprises a plurality of individually-controllablelight sources.
 14. A display according to claim 13, wherein theindividually-controllable light sources comprise light-emitting diodes.15. A display according to claim 10 wherein: the modulator comprises aplurality of pixels each having a controllable light transmission; andthe diffuser sheet comprises a plurality of the diffuser elements withina projected area on the diffuser sheet of each of the plurality ofpixels.
 16. A display according to claim 13 comprising a controllerconfigured to control intensities of the individually-controllable lightsources to provide a low resolution version of an image defined by imagedata on the modulator.
 17. A display according to claim 16, wherein thecontroller is configured to control the pixels of a modulator to displaythe image for viewing at a resolution higher than that of the lowresolution version of the image.
 18. A diffuser sheet according to claim1, wherein the length of the waveguide is at least two times as long asthe diameter of the waveguide.
 19. A diffuser sheet according to claim1, wherein the waveguide is sufficiently long that light rays aretotally internally reflected within the waveguide at least 3 times inbeing carried from the concentrator to the inverse concentrator.
 20. Anoptical element, comprising: an array of diffusion elements disposed ona sheet having a light-incident side and a light-emitting side oppositethe light-incident side; each diffusion element comprising, aconcentrator configured to receive light incident at the light-incidentside, an inverse concentrator configured to emit light from thelight-emitting side wherein the concentrator and inverse concentratorare spaced apart from one another; and an internally-reflectingwaveguide optically coupled between the concentrator and the inverseconcentrator the waveguide configured to carry light from theconcentrator to the inverse concentrator.
 21. An optical elementaccording to claim 20, the optical element further comprising acollimator for collimating incident light rays to be directed withinabout 25° of a direction normal to the sheet, wherein: the concentratoris a compound concentrator; the compound concentrator concentratesincident light rays from the collimator into the waveguide; and thewaveguide decollimates the concentrated light rays and optically couplesthe decollimated light rays into the inverse concentrator.
 22. Anoptical element according to claim 20, wherein the waveguide comprisesan optically-transparent dielectric material surrounded by air.